NASA has awarded Boeing US$425 million towards building and testing a full-sized prototype of its transonic truss-braced wing airliner concept, using long, thin, strut-braced wings to add lift, reduce drag, and burn an impressive 30% less fuel.:
The idea takes advantage of the higher lift and lower drag you get with longer, slimmer, high aspect ratio wings — the sort you might find on an unpowered glider. A concept Boeing was testing in 2016, for example, had wings some 50% wider than comparable standard aircraft.
Structurally, that kind of thing simply doesn’t work without reinforcement. So Boeing’s design hangs the wings from the top of the fuselage, and braces them with long trusses coming up from the belly of the plane. These too are carefully shaped airfoils, adding extra lift as well as strength and stability.
As a subsonic concept cruising at around Mach 0.70 to 0.75 (519 to 556 mph, 835 to 895 km/h), Boeing estimated these braced-wing airliners could burn 50% less fuel than a regular plane. In 2019, the concept was redesigned to cruise at the edge of transonic speed, around Mach 0.8 (593 mph, 955 km/h), and whether because of the added speed or simply from a better understanding of the aerodynamics, Boeing has walked the efficiency claims back.
And then there’s the fact that the huge, thick, lower aspect ratio wings on standard airliners create a perfect hollow space for their fuel tanks. Keeping the fuel out in the wings places a lot of weight out wide, closer to the center of lift, reducing engineering stresses where the wings meet the body. It contributes to safety somewhat in a crash, keeping burning fuel further from the passengers. And from a pure brass-tacks perspective, it frees up room in the cabin for extra money-making seats. The truss-braced design uses such slim wings that fuel tanks will likely have to go back into the fuselage.
Looks like a fancy biplane to me.
Some years ago (pre-Covid), there was a study which showed that some large proportion of air freight was carried in the bellies of passenger planes. Certainly, watching at airports, one can see lots of cargo being loaded into wide-body passenger planes.
If the fuel has to go into the belly of the plane instead of into the wings, there will be a large reduction in cargo capacity and a similar loss in cargo revenue. How does that revenue loss compare to the expense reduction of lower fuel use?
Fuel is really dense and wouldn’t take up a large portion of the cargo space. The more difficult question is: how will the bigger wingspan fit into the existing gates at airports?
“50% wider”
They won’t fit in existing passenger terminals or most hangars, then. And airports are expensive real estate.
A biplane configuration can allow for extra-ordinarily longer lifting surfaces by having them angle to meet at their tips, forming a triangulated structural element which closes the loop. Can involve multiple pairs from front to back coming together as well.